One of the biggest difficulties in performing ground-penetrating radar (GPR) from an elevated platform is the large surface reflection at the air-ground interface. When an electromagnetic wave impinges on the ground (the earth) from above, only part of the wave penetrates the earth and the remainder is reflected.
GPR performance depends both on the rate of attenuation of the signal in the ground and on the reflection coefficient at the surface. The reflection coefficient is dependent on a number of factors. These include the operating frequency, the wave polarization, the ground properties, and the wave angle. Once in the earth, the wave undergoes attenuation that is set by the ground dielectric and conductivity properties and the operating frequency.
Attenuation in the ground increases with the electrical conductivity of the soil or rock and is frequency dependent, with higher frequencies suffering higher attenuation rates. Electrical conductivity in turn increases with clay content and dissolved solids in the soil moisture or ground water.
Upon reflection from a subsurface object, the reflected wave is again attenuated by propagation and reflection at the earth-air interface on the way back to the radar antenna. These losses strongly affect the signal return because they happen twice: on entry and on exit.